1. Field of the Invention
This invention relates to collimated radiation detector assemblies, arrays of collimated radiation detectors and collimated radiation detector modules.
2. Background Art
The material requirements for a room temperature operated high resolution semiconductor gamma ray spectrometer include large free charge carrier mobilities (xcexc), or alternatively, high achievable free charge carrier velocities (xcexd), long mean free drift times (xcfx84*), a relatively large energy band gap (Eg) generally between 1.4 eV to 2.5 eV, high representative values of atomic number (Z), and availability of large volumes. Presently, no semiconductor has all of the listed ideal material properties desired for the xe2x80x9cperfectxe2x80x9d room temperature operated semiconductor radiation spectrometer, although many have a considerable fraction of the required properties. Some wide band gap compound semiconductors that offer promise as room temperature operated gamma ray spectrometers include GaAs, HgI2, PbI2, CdTe, and CdZnTe. One difficult problem to resolve with these materials is gamma ray energy resolution degradation from charge carrier trapping losses.
The general planar detector design that is used for compound semiconductor radiation detectors consists of a block of material with contacts fabricated on either side of the block. Spectroscopic measurements of gamma radiation interactions require that both electrons and holes be extracted efficiently from a conventional planar detector, hence the device dimensions are usually tailored to reduce trapping effects from the most effected charge carrier (usually holes). Generally, compound semiconductors have notable differences between the mobilities and mean free drift times of the electrons and holes. For instance, CdZnTe material has reported mobility values of 120 cm2/V-s for holes and 1350 cm2/V-s for electrons. Additionally, the reported mean free drift times are 2xc3x9710xe2x88x927 s for holes and 10xe2x88x926 s for electrons. Hence, the effect of trapping losses is much more pronounced on holes than on electrons, and the device dimensions would have to be designed to compensate for the problem.
A similar situation is experienced with gas filled ion chambers, in which electron-ion pairs are produced by gamma ray interactions in the gas. The electron mobilities are much higher than the positive ion mobilities, hence the extraction times of the electrons are considerably less than the extraction times of the ions. For typically used integration times, the measured pulse amplitude becomes dependent on the initial gamma ray interaction location in the ion chamber. As a result, wide variations in pulse amplitude are possible. The problem was significantly reduced by Frisch with the incorporation of a grid in the ion chamber near the anode. The measured pulses from the detector corresponded to only the movement of mobile charges in the region between the grid and the anode, hence ion movement in the bulk of the device no longer affected the signal output.
The Frisch grid concept has been demonstrated with semiconductor detectors using a xe2x80x9cco-planarxe2x80x9d design. The devices work well, but unlike the true Frisch grid, they generally require more than one output signal or a circuit capable of discerning the different grid signals.
A simple planar semiconductor detector is operated by applying a bias voltage across the bulk of the material. Ionizing radiation excites electron-hole pairs that are drifted apart by the device electric field. Electrons are drifted towards the anode and holes are drifted towards the cathode. An induced charge is produced at the terminals of the device by the moving free charge carriers, and the induced charge can be measured by an externally connected circuit. Shockley and Ramo derived the dependence of the induced current and induced charge produced by point charges moving between electrodes, which was later shown to apply to semiconductor detectors as well.
The Shockley-Ramo theorem shows that the induced charge appearing at the terminals of a planar device from moving point charges is proportional to the distance displaced by the moving point charges, regardless of the presence of space charge. Hence, the change in induced charge Q* can be represented by                                           Δ            ⁢                          xe2x80x83                        ⁢                          Q              *                                =                                    Q              o                        ⁢                                          |                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                                      x                    e                                                  |                                  +                                      |                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              x                        h                                                              |                                                                              W                D                                                    ,                            (        1        )            
where Qo is the initial charge excited by the interacting gamma ray, WD is the detector length, xcex94x is the distance traveled by the electrons or holes, and the e and h subscripts refer to electrons or holes, respectively. With trapping, the total induced charge from a single gamma ray event in a planar semiconductor detector can be represented by
Q*=Qo{xcfx81e(1xe2x88x92exp[(xixe2x88x92WD)/xcfx81eWD])+xcfx81h(1xe2x88x92exp[xe2x88x92xi/xcfx81hWD])},xe2x80x83xe2x80x83(2) 
where xi represents the interaction location in the detector as measured from the cathode and p is the carrier extraction factor represented by                                           ρ                          e              ,              h                                =                                                    v                                  e                  ,                  h                                            ⁢                              τ                                  e                  ,                  h                                *                                                    W              D                                      ,                            (        3        )            
where v is the charge carrier velocity and xcfx84* is the carrier mean free drift time. From equations 2 and 3, it becomes clear that the induced charge (Q*) will be dependent on the location of the gamma ray interaction. Small values of xcfx81 for either holes or electrons will cause large deviations in Q* across the detector width. The induced charge deviation can be greatly reduced if a detector is designed such that the carrier with the longer mean free drift time and highest mobility contributes to all or most of the induced charge.
A Frisch grid gas ion chamber is designed to measure the induced charge primarily from electrons, and the general configuration and operation of a Frisch grid ion chamber is shown in FIGS. 1a and 1b. A gamma ray interaction occurring in the main volume of the detector excites electron-ion pairs. An externally applied electric field drifts the carriers in opposite directions, in which the electrons drift through the grid and into the measurement region of the device. From the Shockley-Ramo theorem, the induced charge produced at the anode results from charge carriers moving between the grid and the anode and not from charge motion between the cathode and the grid. As a result, the detector is primarily sensitive to only the electron charge carriers.
A simple semiconductor Frisch grid detector can be built using the design shown in FIG. 2. As shown, a semiconductor block is cut and polished with metal electrodes fabricated at the ends. These electrodes serve as the anode and cathode. Parallel metal contacts are fabricated on opposite faces of the device, which serve to act as the Frisch grid. The region between the cathode and the parallel Frisch grid is the interaction region, the region underneath the parallel grid is the pervious region, and the region between the parallel grid and the anode is the measurement region. The device is a three terminal device, with the electrodes biased such that electrons are drifted from the interaction region, through the pervious region between the parallel grid, and into the device measurement region.
The different regions and their designations are again shown in FIG. 3. A gamma ray event occurring in the interaction region will excite electron-hole pairs. Electrons are swept from the interaction region towards the parallel grid, however some trapping will occur as the electrons drift across the interaction region and the measurement region. Including the effect of trapping, the measured induced charge from electrons excited in the interaction region by a gamma ray event at a distance xi from the cathode will be                                           Q            *                    =                                    K              ⁡                              (                                  x                  ,                  y                                )                                      ⁢                          Q              o                        ⁢                                          ρ                                  e                  ⁢                                      xe2x80x83                                    ⁢                  m                                            (                              1                -                                  exp                  ⁡                                      [                                                                  -                        1                                                                    ρ                                                  e                          ⁢                                                      xe2x80x83                                                    ⁢                          m                                                                                      ]                                                              ⁢                              xe2x80x83                            )                        ⁢                          exp              [                                                                    x                    i                                    -                                                            W                      p                                        2                                    -                                      W                    i                                                                                        v                    e                                    ⁢                                      τ                    *                                    ⁢                  e                                            ]                                      ,                            (        4        )            
where K(x,y) is a correction factor for deviations in the weighting potential across the device and                                           ρ                                          e                ⁢                                  xe2x80x83                                ⁢                m                            ,                              h                ⁢                                  xe2x80x83                                ⁢                m                                              =                                                    v                                  e                  ,                  h                                            ⁢                              τ                                  e                  ,                  h                                *                                                    (                                                                    W                    p                                    2                                +                                  W                  m                                            )                                      ,                            (        5        )            
where the symbols are shown in FIG. 3. It is assumed that the induced charge on the anode begins to increase primarily as the electrons transit across the middle of the pervious region. For gamma ray interactions that occur directly in the measurement region, the induced charge will now be dependent on both electron and hole motion within the measurement region. Including the effects of electron and hole trapping, the induced charge from gamma ray events occurring in the measurement region is                               Q          *                =                                            Q              o                        ⁢                          K              ⁡                              (                                  x                  ,                  y                                )                                      ⁢                                          ρ                                  e                  ⁢                                      xe2x80x83                                    ⁢                  m                                            (                              1                -                                  exp                  ⁡                                      [                                                                  (                                                                              x                            i                                                    -                                                      W                            D                                                                          )                                                                                              ρ                                                      e                            ⁢                                                          xe2x80x83                                                        ⁢                            m                                                                          ⁡                                                  (                                                                                                                    W                                p                                                            2                                                        +                                                          W                              m                                                                                )                                                                                      ]                                                              ⁢                              xe2x80x83                            )                                +                                    Q              o                        ⁢                          K              ⁡                              (                                  x                  ,                  y                                )                                      ⁢                                          ρ                                  h                  ⁢                                      xe2x80x83                                    ⁢                  m                                            (                              1                -                                  exp                  ⁡                                      [                                                                  (                                                                              W                            D                                                    -                                                      x                            i                                                    -                                                      W                            m                                                    -                                                                                    W                              p                                                        2                                                                          )                                                                                              ρ                                                      h                            ⁢                                                          xe2x80x83                                                        ⁢                            m                                                                          ⁡                                                  (                                                                                                                    W                                p                                                            2                                                        +                                                          W                              m                                                                                )                                                                                      ]                                                              ⁢                              xe2x80x83                            )                                                          (        6        )            
The device is designed such that the measurement region is considerably smaller than the interaction region. Assuming fairly uniform irradiation of the device (for instance, from the detector side), the fraction of events occurring in the interaction region can be approximated by                                           F            i                    ≈                                                    W                i                            +                                                W                  p                                2                                                    W              D                                      =                                                            2                ⁢                                  W                  i                                            +                              W                p                                                    2              ⁢                              (                                                      W                    i                                    +                                      W                    p                                    +                                      W                    m                                                  )                                              .                                    (        7        )            
Semiconductor Frisch grid detectors based on the side grid design have been demonstrated as viable detectors. The devices show improved results over the simple planar detector designs, and they perform well with only one preamplifier output per device. As a result, the semiconductor Frisch grid is a much simpler device to operate and manufacture, much more so than co-planar or micro-pixelated devices.
Semiconductor-based imaging arrays offer improved performance over present scintillator-based imagers (such as Anger cameras) due to their energy higher resolution and the ability to make small detector arrays. Since HgI2 and CdZnTe materials can be operated at room temperature, the operation of semiconductor imaging arrays can be significantly simplified. Unfortunately, most room temperature operated semiconductor materials that are attractive for gamma ray spectroscopy suffer from charge carrier trapping losses, hence the gamma ray energy resolution is greatly compromised unless a resolution enhancing technique is used.
Present methods under investigation for semiconductor-based gamma ray imaging arrays generally involve the use of large blocks of semiconductors upon which numerous detector pixels have been fabricated. Since the devices essentially share the same bulk material, signals induced by charge motion can cause shared signals between adjacent pixels, which works to decrease the spatial resolution. Additionally, energy resolution enhancement is accomplished primarily through virtual Frisch grid techniques, such as with co-planar electrode designs and the xe2x80x9csmall pixel effect.xe2x80x9d Such schemes often require complicated electronic readouts that add to the manufacturing expenses.
Due to problems with scattered gamma rays blurring the images, heavy metal collimators are often used in conjunction with an imaging detector. The collimator significantly reduces the detection of gamma rays that originate or scatter from locations that are not directly aligned with the collimator. These collimators are almost always attached to the detector array after the device array has been constructed.
U.S. Pat. No. 6,175,120 to McGregor et al. discloses a high resolution, solid state, ionization detector and an array of such detectors.
The following U.S. patents are also relevant: U.S. Pat Nos. 5,847,398; 5,627,377; and 5,587,585.
An object of the present invention is to provide a high resolution collimated radiation detector assembly, an array of collimated radiation detectors and a collimated radiation detector module.
In carrying out the above object and other objects of the present invention, a collimated radiation detector assembly is provided. The assembly includes a high resolution radiation detector including an ionization substrate having first, second and third surfaces. The second surface opposes the first surface and the third surface is located between the first and second surfaces. The detector further includes a first electrode disposed at the first surface and a second electrode disposed at the second surface. The assembly also includes a structure having a housing with a compartment for housing the detector. The structure also has a conductive collimator aligned with the housing for collimating radiation to the detector. The housing divides the substrate into interaction, measurement and pervious regions.
The housing and the collimator may form a single structure or may be separate structures which are connected together.
The housing may act as a grid such as a Frisch grid.
The measurement region of the substrate may extend out of the housing and the pervious and interaction regions may extend into the housing.
The length of the compartment may be greater than the length of the pervious and interaction regions.
The radiation detector may be a semiconductor radiation detector such as a single-charge carrier radiation detector.
The radiation detector may be a planar semiconductor detector or a room temperature, gamma ray or x-ray detector.
The assembly may further include insulating material disposed between the housing and the third surface in the compartment to insulate the housing from the third surface.
The assembly may further include a member disposed in the compartment of the housing for coupling the second electrode to the housing. The member may be conductive to electrically couple the second electrode to the housing.
The detector may be a pixelated detector having a plurality of separate detector portions and a plurality of electrodes disposed thereon. Further, the housing may have a plurality of separate compartments for housing the detector portions.
The substrate may have tapered between the first and second surfaces to provide geometric weighting to the detector.
The first electrode may be substantially smaller than the second electrode to improve detected radiation energy resolution.
The assembly may further include a shield attached to the collimator to shield the detector from electromagnetic noise.
Further, in carrying out the above objects and other objects of the present invention, an array of collimated radiation detectors is provided wherein each of the detectors is a detector assembly as described above.
The array may be an imaging array such as a gamma ray or x-ray imaging array. The imaging array may be a semiconductor-based imaging array.
Still further in carrying out the above objects and other objects of the present invention, an array of collimated radiation detectors is provided. The array includes a plurality of high resolution radiation detectors wherein each of the detectors includes an ionization substrate having first, second and third surfaces. The second surface opposes the first surface and the third surface is located between the first and second surfaces. Each of the detectors further includes a first electrode disposed at its first surface and a second electrode disposed at its second surface. The array also includes a structure including a housing having a plurality of separate compartments for housing the detectors. The structure also includes a conductive collimator aligned with the housing for collimating radiation to the detectors. The housing divides each of the substrates into interaction, measurement and pervious regions.
The housing and the collimator may be formed from a single structure of high-density material or the housing and the collimator may include an array of sheets of high-density material for separating adjacent detectors. The sheets may be corrugated.
The array may be an imaging array such as a gamma ray or x-ray imaging array. The imaging array may be a semiconductor-based imaging array.
Each of the substrates may be tapered between its first and second surfaces to provide geometric weighting to the detectors.
The first electrode of each of the detectors may be substantially smaller than the second electrode to improve detected radiation energy resolution.
The array may include a shield attached to the collimator to shield the detectors from electromagnetic noise.
The housing and the collimator may include at least one slotted structure of high-density material.
Yet further in carrying out the above object and other objects of the present invention a collimated radiation detector module is provided. The module includes an array of collimated radiation detectors having a plurality of high resolution radiation detectors. Each of the detectors includes an ionization substrate having first, second and third surfaces, the second surface opposing the first surface and the third surface being located between the first and second surfaces. Each of the detectors further includes a first electrode disposed at its first surface and a second electrode disposed at its second surface. The array further has a structure including a housing having a plurality of separate compartments for housing the detectors. The structure also includes a conductive collimator aligned with the housing for collimating radiation to the detectors. The housing divides each of the substrates into interaction, measurement and pervious regions. The module further includes circuitry for collecting signals from the detectors and a lid connected to the structure for covering the circuitry.
The module may further include soft conductive material for establishing electrical connections with the first and second electrodes of the detectors.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.